"I grew up in the negative about Winston Churchill," Brain Cox says. "He was MP of my home town, Dundee. We didn't particularly like him. But the war changed everything."

Cox is in Auckland discussing his title role in the biopic Churchill. It's an astonishing performance that captures a Churchill you've never seen before.

"There's so many aspects to the man in the film. More than you normally find in a Churchillian story. [There's] the public Churchill and then there's the private Churchill - the one that's depressed, the black dog, suffering from depression. And the one that's extremely nervous about D-Day. About Operation Overlord. He's not totally convinced by it."

The film takes place in those stressful, uncertain days leading-up to the D-Day invasion of World War II, a battle that was a huge gambit for the allies yet absolutely crucial in bringing an end to the war.

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It was also an operation that Churchill was bitterly opposed to.

"Now historians are going to argue with that," Cox concedes. "They're gonna say, 'it's this, that and the next thing'. But the truth of the matter is that for many, many years this never came out. It didn't become public knowledge until quite recently."

The film spends a lot of time in London's bunker war room, where an increasingly agitated Churchill wrestles with depression, drinking, his temper and his inability to convince the allies leadership, chiefly the plan's American architect General Eisenhower, that Operation Overlord is far too risky and carries a potentially tremendous cost.

"He'd been through the Dardanelles," Cox says, referencing Churchill's similar plan of a naval-led, beachfront invasion in World War I, which led to the catastrophic Gallipoli battle.

"The Dardanelles could have been a success. There was a problem with the officers, there was a problem with mismanagement. But it was very ambitious and it didn't work. A quarter of a million men were lost. Australian's, New Zealanders ... it's particularly pertinent to the Antipodes. And I think it's something that never left him. Especially if you take the view that amphibious landings are based on the fact that the first line of attack usually becomes obliterated. Something like 1500 men died within the first 20 minutes. I don't care what anybody says, that has to impinge on you."

The film shows just how haunted Churchill was by his plan's failure.

"He knew about the cost and I think with the best will in the world he wasn't inured to the human cost. As he was getting older I think it really got to him."

Brian Cox stars as Winston Churchill in new film Churchill
Brian Cox stars as Winston Churchill in new film Churchill

"[WWII] was a hell of a responsibility for him. The whole campaign. He was aged 70. He didn't sleep, [only] four hours a night. The pressure on him was phenomenal. He'd been ill. At the end of 1943 he had pneumonia. He wasn't a man who was in the best of health. He certainly wasn't fit. He drank phenomenally, Champagne for breakfast, whisky for lunch and brandy for supper. Apart from all the wine he'd consume in between as well. He was a bon vivant. That's what he was. He was a life lover. But I think by 1944 - and this is another key factor - he was pretty exhausted."

In order to convince his allies not to go ahead with Overlord, the film shows Churchill masterminding an alternate plan, which is rebuked in favour of carrying on with Overlord.

"Two thirds of the way through our shoot, the military adviser, he happened to say, 'you know, we put Churchill's alternative plan into the computer 30 years ago at Sandhurst,' Cox recalls.

"I said, 'oh really? What was the result?'. And he, a little shamefacedly went, 'Well, given a few little moderations, the war would have ended six months earlier'."

Cox smiles at the story and says, "On top of everything else ... it's interesting. He was right. He's a fascinating character. The war was the thing that made him the man of destiny. He was an extraordinary figure. And he rose to the occasion brilliantly."